1. Field of the Invention
The present invention relates to a light homogenizing optical sheet used to create uniform light, a method of manufacturing the optical sheet, and a backlight system that uses the optical sheet for a display device.
2. Description of the Related Art
While a variety of solid-state display devices have been developed in the past, Liquid Crystal Displays (LCDs) are attractive due to low cost, reliability, low power and voltage requirements, longevity, and availability. Typical fluorescent backlights have high voltage requirements and relatively short lifespans. In addition, much of the light from fluorescent backlighting exhibits high angular frequencies that can contribute to scatter, potentially reducing display system contrast.
Due to the advantages of solid-state performance, reduced size, low voltage and power requirements, long life, and the increased performance in color gamut, light emitting diodes (LEDs) have gained attention for use in display applications. Arrays of light sources, such as LEDs, have been used in solid-state displays, but typically, a diffuser is positioned adjacent to the LEDs to redistribute the light uniformly across the area of interest. U.S. Pat. No. 5,499,120 discloses an alternative method of diffusing light from the liquid crystal material in a particular Double Super Twisted Nematic cell of a LCD. According to the inventor, the LCD panel provides sufficient light spread such that a diffuser is no longer needed, reducing losses in efficiency caused by the diffuser. However, it should be noted that uniform output at a given distance from an array of sources improves as the sources are spaced at a closer pitch. As price/performance ratios drop with the introduction of new brighter LED sources, it is desirable from a cost standpoint to utilize such sources to minimize the number of source elements needed in a particular application. Due to this, the uniformity of the backlight is an important key to high performance products with low cost. There remains a need for a highly efficient diffuser that can efficiently utilize the light from the array of sources in order to keep costs down and performance high.
U.S. Pat. No. 5,816,681 discloses a system that uses an optical diffuser that can accept light from a source at a specific direction and emit it in a desired viewing direction. The system uses a plurality of LEDs and a holographically recorded optical diffuser with low backscatter and high transmission. Uniform illumination can be utilized in various fields, including the automotive industry and the illumination industry.
Microlens arrays have gained significant interest in backlighting applications. U.S. Pat. No. 4,924,356 discloses a backlight that uses a plurality of microlenses to collimate light emanating through pinholes from a light box. Substantially collimated and uniform backlight can be realized using such a system, however, light efficiency can be affected.
FIG. 1 is an illustration of optical system in the prior art that uses two optical sheets each with a microlens array formed on one surface. The optical sheets are aligned so that the microlens arrays are facing each other and registered. They are spaced apart by the focal length of the lenslet focal length. One drawback with using two optical sheets with microlens arrays formed on one surface is that they must be relatively aligned to a high degree of accuracy, limiting the use of dual MLA homogenizer to high-cost applications having assemblies with more complex mounting techniques in order to align and keep registration. Another drawback is limited acceptance angle that can be achieved, due to Total Internal Reflection (TIR) limitations of the first array. A third drawback is scatter caused by the TIR limitations. A fourth drawback is limited exit angle that can be achieved due to TIR limitations of the first array as well as realistic limitations due to refractive index relationships.
Warren Smith in “Practical Optical System Layout” McGraw-Hill 1997 pages 106–107 illustrates the use of a homogenizer array of microlenses 7 in order to even out the intensity from a projection lamp source 5 collimated by a parabolic mirror 6 so as to illuminate a Dual MLA so that the light seen at the film gate is substantially uniform across the film plane 9 imaged by lens 8, as illustrated in FIG. 1. It should be noted that such a system only allows uniformity versus position across the film plane and does not address uniformity versus angle within the film gate.
In Optical Engineering (vol. 41 no. 10 Oct. 2002 pages 2393–2401), Buttner et al confirms how microlens arrays can be utilized to homogenize light from a single LED source. This paper confirms that microlens arrays can be used in a typical Fourier transform setup such that light from a source diverges, is collimated by a first lens to illuminate a microlens array, then a second Fourier transforming lens is placed such that the microlens array is substantially located at the front focal length of the second lens. The plane at the back focal length of the second lens will then exhibit substantially uniform light. By placing two microlens arrays in tandem spaced apart by the distance of the focal length of a lenslet in the array, the uniformity at the output plane of the system described can be substantially improved. Although not obvious, two opposingly faced microlens arrays in tandem at the focal length separation distance have inherent design limitations on exit Numerical Aperture extent due to the effect of total internal reflection. Light illuminating the first lenslet beyond the critical angle cannot be transmitted toward the second lenslet. In addition, uniformity versus angle at the output plane is not achieved with such system.
A self-limiting isotropic wet etch process is disclosed in U.S. Pat. No. 6,379,573. The publication proposes forming a spherical cavity in a substrate while using tape as an etch mask. It suggests that dimensions of the cavity, such as the radius of curvature, as well as uniformity, can be precisely controlled.
U.S. Pat. No. 6,363,603 discloses a method for manufacturing an erect image of unity magnification, resin lens array by injection molding. The best mode for carrying out the invention includes the step of forming a master by isotropic etching a homogenous substrate through a mask of pinholes. Such a method exhibits high quality results in terms of the accuracy and sharpness at the seams or transition points between lenslets, but the inventors of this present invention have found that substantially isotropic etching can form flat-top profiles at the apex of each lenslet center. Also, substantially aspheric profiles for high numerical aperture lenslets are not possible using the method of isotropic etching through pinholes.
U.S. Pat. No. 6,411,439 discloses a microlens array manufacturing method that involves forming a master, preferably by isotropic etching a homogenous substrate through a resist mask, forming an intermediate master of the original master, forming a duplicate master from the resulting intermediate master, and then forming a microlens array from the duplicate master. It suggests that this method reduces the frequent need of high cost masters thus enabling low cost manufacturing of microlens arrays.
Since high efficiency places some importance on fill factor of the lenslets in a microlens array and high numerical aperture microlens arrays can be desirable for backlighting applications, there is a need for a manufacturing method to achieve both substantially aspheric profiles as well as sharp lenslet seams.